Filters made from various filtration media and shapes are employed as a particulate pollution control approach for many processes. Typically, many filters are packaged into a housing that may include a few filters to over 10,000 filters. As the filters collect dust from the process, pressure drop builds up across the filter to the point where the filters need to be cleaned. Common cylindrically-shaped bag filters are typically cleaned by a pulse of high-pressure air injected into the top of the filter, which will dislodge much of the dust collected on the outside of the filter elements and allow the dust to fall to a collection hopper. Filters may also be cleaned by mechanical shaking or with a low-pressure, reverse-air mechanism. The filters are typically cleaned as often as every few minutes to as long as several hours. With multiple cleanings, the filters develop high pressure drop over time because of a thin but tenacious amount of residual dust that does not clean off by normal filter cleaning methods. If the pressure drop is too high and the residual dust will not clean off, the filters are said to be blinded. In many cases, the filters have to be replaced, not because they are worn, but because of the high pressure drop due to filter blinding.
To minimize the size of a filter housing containing filters, it is desirable for it to operate at a high filtration velocity, called air-to-cloth ratio (A/C ratio). One of the biggest obstacles to operation of large-scale filter housings at high A/C ratios is removing the residual dust from the filters. The general term for the flow resistance due to the dust left on the filters after normal cleaning is “residual drag,” which is simply the pressure drop across the filter divided by the filtration velocity. For a fabric type filter operating at an A/C ratio of 12 ft/min that has an after-cleaning pressure drop of about 6-in. W.C., the residual drag would be 0.5-in. W.C./ft/min. This compares with a new fabric type filter having a residual drag of only about 0.1-in. W.C./ft/min for a typical membrane fabric. Many times filter residual drag may reach a value over 1.0-in. W.C./ft/min, which means significant power is required by a fan maintaining gas flow overcoming the high residual drag of a filter. Further, the filtering process may also be limited by the fan's capacity to draw gas flow through the filter. The major incentive to operate these filters at as high of an A/C ratio as possible is economic advantage.